In multiple input multiple output (MIMO) communications systems, multiple antennas are used on both the base station and on the receiver or transmitter that communicates with the base station to exploit a phenomenon known as multipath propagation in order to achieve higher data rates. In general, MIMO communications systems simultaneously send and receive multiple data signals over each radio channel. The multipath propagation phenomenon is the result of environmental factors that influence the data signals as they travel between the base station and the transmitter or receiver, including, for example, ionospheric reflection and refraction, atmospheric ducting, reflection from terrestrial objects and reflection from bodies of water. Because of these factors, the data signals experience multipath interference that results in constructive interference, destructive interference, or fading, and phase shifting of the data signals. MIMO technology has been standardized in various wireless communications standards including Institute of Electrical and Electronics Engineers (IEEE) 802.11n, IEEE 802.11ac, HSPA+ (3G), WiMAX (4G) and Long Term Evolution (LTE) standards.
MIMO communications systems require testing. A typical MIMO test system for testing a device under test (DUT) includes a base station emulator, a fading emulator, a personal computer (PC) that functions as a test instrument, some type of multi-probe antenna configuration, and various electrical cables for interconnecting the components. In some MIMO test systems, the output ports of the fading emulator are connected to the antenna ports of the DUT by electrical cables. This type of MIMO test system is known as a cable-conducted MIMO test system. Disadvantages to this type of MIMO test system include having to break open the DUT to access the antenna ports of the DUT, unavailability of DUT antenna ports in some cases, and the need to take active antenna effects into account.
Another type of MIMO test system that is used to test DUTs is a multi-probe anechoic chamber (MPAC) over-the-air (OTA) test system. In a typical MPAC OTA system, the DUT is located inside of an anechoic chamber that includes a multi-antenna probe configuration. The output ports of the fading emulator are connected to the respective antenna probes of the chamber.
Another known MIMO test system uses a radiated two-stage (RTS) methodology. The test set up is similar to that of the MPAC OTA set up. In the first stage, the radiation pattern of the DUT is determined based on signal power and relative phase reported by the DUT to the test instrument. In the second stage, the DUT is placed inside of another chamber that is equipped with a plurality of probe antennas. During the second stage, calibration is performed to measure the radiation channel matrix for the OTA channel between the probe antennas and the antennas of the DUT. The inverse matrix of the radiation channel matrix is calculated and multiplied by the channel model being emulated by the fading emulator. A disadvantage of this test system is that the radiation channel matrix includes values associated with properties of the DUT antennas, and it is limited to cases where the radiation channel matrix is not greater than a 2-by-2 matrix. For this reason, the inverse matrix cannot be accurately measured for all cases, and therefore the test system cannot be accurately calibrated. Consequently, the DUT performance measurements obtained by the test system are not as accurate as they should be.
A need exists for a robust MIMO OTA radiated test system that eliminates the need to make wired connections to the antenna ports of the DUT, that is capable of accurately measuring the radiation channel matrix so that testing can be more accurately performed, and that is not limited with regard to the dimensions of the radiation channel matrix.